Two babies that are physically stuck together, physically connected are called Conjoined Twins, also known as Siamese Twins. They are also called Siamese Twins due to Conjoined Twin men that worked on the circus. Due to the rare condition and the expression “Siamese Twins”, they made that the synonym for Conjoined Twins.
When a single fertilized egg splits and develops into two fetuses, the result is Identical Twins. The split occurs during the first 12 days after conception. When the fertilized egg splits later than 12 days, typically among 13 and 14 days after conception, separation stops. The zygote did not split totally. Separation stops before the process is complete, so Conjoined Twins are formed. This theory is called the “fission theory.” How and where the twins will be joined is determined when the split of the egg happens. Another theory suggested that the cause for this disorder is that two separate embryos may somehow combine together during early development. Some scientists have argued that the fertilized egg completely separated but the stem cell is what fused then together. Still, no one knows what might be the cause for both theories.
Conjoined or Siamese Twins occur one in 100.000 births or one in 500,000 babies. This can be identified via ultrasound at an early stage of pregnancy. Conjoined Twins share placentas and membranes, but can also share one or more organs and body parts. They are mostly joined at the head, pelvis and chest. Siamese Twins are more often shown on women than men, with a ratio of 3:1. 75% of the Conjoined Twins are females and 70% are joined together at the thorax or abdomen. Most Siamese Twins die after birth or are stillborn. 40-60% are still born, and many other die after few days of birth. The percentage of survival is about 25%.
There are fifteen types of Conjoined Twins: Thoracopagus, Omphalopagus, Xiphopagus, Ischipagus, Ischio-Omphalupagus, Parapagus, Dicephalus, Craniopagus, Cephalopagus, Cephalothoracopagus, Diprosupus, Pygopagus, Rachipagus, Acardiac Twins, and Parasitic Twins.
In some cases, some Conjoined Twins can be separated with a surgery. It depends on where the twins are joined and how many organs are shared for the surgery to be successful. Also, it depends on the experience of the doctor and its team. This is a risky surgery, and in most cases the result is death of both or one of the twins.
An article has discussed cases where there were conjoined triplets, and even quadruplets. The article specifically showed a case of Paragus triplets joined with a shared sternum. All three babies were well formed and had normal heads and extremities. This is exceedingly unusual, but may happen.
The separation has been practiced since 1950’s. Although the first successful surgery was in Switzerland in 1689, twin girls that were joined by a ligament. The first successful separation after the time it has been attempted (1950’s), was in 1953. They were two girls from Louisiana and were joined together in the lower back at 8 days of age.